The sport of bicycle motocross (or BMX) involves the use of special bikes to imitate motorcycle motocross riders. BMX is a large industry that has bikes in the low end price range that are purchased in department stores and rarely see dirt riding, bikes that are purchased in the mid-price range at bicycle shops for street, dirt and racing action, and bikes at the high priced end that are purchased at bicycle shops and used strictly for BMX racing.
Chain drive systems for bicycles have several deficiencies. When at rest, there is slack in the chain as well as slack between the individual chain links. When the rider begins to pedal, there is a lag time before the rear wheel begins moving since the slack in the chain plus the slack between the individual links has to be taken up. All of this slack moves to the bottom section of the chain. This means that the the bottom section of the chain is simply being carried along by the drive system without contributing driving force to the system.
The use of belt driven systems as drive units has proven to be much more efficient than chain drives in industrial applications. Belts are lighter than conventional chains and they require no oil or grease and are maintenance free. Further, there is no noise that is associated with chain drives and the belt is essentially silent. Belt drives also require torque regulators that constantly balance the slack in the belt. When this is accomplished properly, there is instant engagement/movement of the driven wheel by the driving wheel. With bicycles, this means there is instant engagement with the rear wheel when the rider begins pedaling. There is also added power to the rear wheel on the bottom section of the belt since it is in full driving contact with the teeth of the rear drive wheel pulley. This is known as "tooth wrap". Additionally, the belt has less friction, more surface area over which to transmit torque and less need for force to overcome the wrapping of the belt around the pulleys versus the wrapping of the chain.
Prior art tension regulators include idler wheels such as those disclosed in U.S. Pat. No. 4,768,998 that are close to each other to urge the toothed belt in a direction so as to apply a certain tensile force thereto in order to prevent the toothed belt from being removed or slipping. However, these idler wheels are rigidly mounted on the frame and do not pivot and when the toothed belt has force applied thereto by the idlers during tightening of the belt, continued movement of the belt with unequal force applied to it in its curved condition causes the belt to become damaged and requires the belt to be replaced.
Other devices such as disclosed in U.S. Pat. No. 976,115 utilize idler wheels that are coupled to each other by spring tension, thereby providing tension to the belt. However, the tension is uneven because the spring that connects the idler rollers and cause the adjustment to the belt does not always provide balanced forces to both sides of the belt when tension is applied to the belt.
Still another device disclosed in U.S. Pat. No. 4,034,821 utilizes a chain driven sprocket being pivoted on a swing arm about an axis intermediate the driven sprocket and the driving sprocket and coupled to a suspension system. Idler members on opposite sides of the pivot axis engage and pinch the chain flights together at points equidistant from the pivot axis. Thus, pivotal movement of the driven sprocket about the axis is resisted and there is a tendency for the swing arm to seek an equalized condition. This system is designed for chains and is rather complex and causes a pivotal movement of the driven sprocket about the axis to be resisted by tightening the chain since leverage is applied to the idler members and all moments operating about the pivot axis are applied in a direction opposite to the pivotal movement of the driven sprocket.
U.S. Pat. No. 4,191,062 discloses an automatic drive chain tensioner having a pair of idler assemblies with each idler assembly comprising an idler sprocket and an indexing sprocket on each side of and concentrically connected to the idler sprocket with each idler assembly being adapted to be positioned on the outside of a drive chain loop on a chain run between a driving and a driven sprocket. A pair of indexing chain loops each of which engages an indexing sprocket on each of the spaced-a-part idler assemblies is also required. Again, this system is extremely complex, is designed for chain drives and is expensive to manufacture and maintain.
The present invention overcomes the disadvantages of the prior art by providing a simple unitary tension plate having orifices in the general shape of a triangle with an orifice forming each of the apices of the triangle. One of the orifices is used as a pivot point to couple to a frame member connecting the driving pulley and the driven pulley. A roller is mounted in each of the other orifices as an idler wheel and the belt coupling the driving pulley to the driven pulley passes between the idler wheels to maintain a predetermined tension on the belt. When the upper section of the belt is placed under tension such as when causing movement of the rear wheels, it places a pressure on the upward idler wheel tending to pivot the tensioner plate about its pivot point. This causes the other idler wheel to place pressure against the lower section of the belt.
Thus, because of the close center-to-center distance of the idler wheels, which distance is fixed by the unitary tension plate, preferably triangular shaped, the tight side of the drive belt, when attempting to form a straight line under tension, applies pressure to the idler wheel resting against it and rotates the tension plate. This rotation of the tension plate, in turn, causes the slack side of the belt to be tightened by the other idler wheel pressing against it. Thus, the tension plate automatically attempts to move into a position which seeks a balance of forces on both the upper and lower section of the belt. Thus, the device is very simple, very economical, and very effective in maintaining balanced forces on both sides of the belt around the driven pulley.
It is therefore an object of the present invention to provide a belt tensioning system that is not only simple and economical to construct, but also that is effective in maintaining balanced forces on the driving belt around the driven pulley and maintaining sufficient "tooth wrap" around the driven pulley.
It is also an object of the present invention to provide an automatic tensioning system which includes a triangular shaped tension plate that is pivotally attached at one corner to the frame and which has idler wheels mounted in the other corners of the triangular tension plate and between which the belt moves in its continuous path such that any forces applied to one side of the belt or the other causes the tension plate to automatically move to adjust the pressure on both sides of the belt so that tensions on the belt remain constant and no slack is allowed to exist.